These polycarbonate products include in particular qualitatively high-grade extrudates such as films, sheets or cable sheathings based on polycarbonate. These articles may be produced by conventional techniques, such as for example extrusion, blow molding or press molding methods. The invention also provides a process for the production of such substrate materials.
Extrusion films of polycarbonate, polyester carbonate or also blends of PC and polyesters such as polyethylene terephthalates, polybutylene terephthalates or polycyclohexanedimethanol cyclohexanedicarboxylate (PCCD), such as for example those with the registered trade marks SOLLX® or XYLEX® (both from General Electric, USA), are used especially in the electronics sector, for decorative and functional blends in the domestic appliances sector, as sealing films, e.g. for sports articles, for ID cards and blister packaging. Further areas of application are in the automotive manufacturing sector, such as for example vehicle body parts or exterior mirrors, or in the telecommunications sector, such as for example mobile phone cases and keypads. The films are characterized by high transparency, impact strength and thermal stability. A further critical quality feature is the surface quality of the film. Surface defects readily occur especially with high-grade housing and display coverings, which leads to an increased rejection rate.
High quality extrusion films are also important as cover layers for optical data storage media. Here the requirements as regards the quality of the film are particularly stringent, since defects in the transilluminated cover layer material may lead to errors in the readout process. Thus, in particular defects which may interact with the laser beam of the readout system are of particular relevance as regards an error-free readout process. Such defects include, as is known, foreign particles such as for example dust particles or metal particles, which may absorb and/or scatter the laser beam. Due to the reduction of the wavelength of the readout laser, in addition those particles whose absorption lies within the wavelength of the employed laser are also damaging. For example with the Blu ray Disk, which is equipped with a cover layer, these wavelengths are for example between 400 nm and 410 nm.
It is known that foreign particles may initiate defects in the polymer matrix. Defects in the surface may be caused by a specific location of the foreign particle, for example in an extrusion film. These foreign particles may be carbon particles, so-called “black spots”, dust, metal particles, other polymer impurities (different from the used base resin) such as for example polyamide, or cotton fibers, etc.
Furthermore the surface imperfections may be caused by the extrusion process itself. Depending on the extrusion conditions, such as for example calender settings, defects such as air inclusions may be produced. These defects may be minimized by optimizing the production conditions.
It is known that the number of foreign particles in the extrusion film may be reduced by ensuring particularly clean ambient conditions, such as clean room conditions, for example by using filtered air. This is described for example in DE 19952852.
Furthermore substrate materials with a low foreign particle content and which provide a good surface quality are described for example in JP 2003231141. These substrate materials described in JP 2003231141 are outside the scope of the present invention, since the present invention concerns substrate materials that have a low concentration of a specific type of particles.
In DE 199 52 850 polycarbonate substrates are described that are suitable for the production of molding compounds and films having a high surface quality. In particular these materials are suitable for the production of twin-wall sheets and solid sheets. This substrate material may be produced by specific production processes, in which the starting materials have low concentrations of certain elements such as Fe, Cr, Ni, Zn, Ca, Mg or Al. However, the formation of the particles covered by the present invention and thus the surface defects are not suppressed by reducing the concentration of these elements.
In U.S. Pat. No. 6,288,204 gel particles in branched polycarbonate are described, which may lead to defects. In principle it is known that branched polycarbonate contains defect sites. These are not covered here. The present invention relates to linear polycarbonates.
It was surprisingly found that defects in the surface of the film are caused not only by usual dust particles or other foreign particles, but in particular by fluorescing particles with specific mechanical properties. These particles cannot be separated by conventional filtration. The foreign particles described in the present invention, which lead to excessive surface defects, cannot be detected in a solvent in which the substrate material is soluble, since they have a similar refractive index to polycarbonate. Thus, these particles cannot be detected for example by a Hiac Royco test corresponding to the prior art.
The normally filterable particles known in the prior art, such as for example “black spots”, are therefore not covered by the present invention.
It was found instead that for example 40-80% of the particles that initiate a surface defect contain a fluorescing core. It was furthermore found that the size of these fluorescing particles is between 15 and 250 μm.
It was in addition found that a majority of these fluorescing particles have mechanical properties different from the polymer matrix (polycarbonate) itself. Specific measurements of the mechanical properties of the particles reveal a higher modulus as well as a greater hardness compared to the matrix material (polycarbonate). These fluorescing particles may initiate defect sites, such as for example depressions, pits or round deformations or concavities, on the film surface, which significantly reduce the quality of the film. These defect sites then lead to problems particularly if the film is to be coated or metallized with a further material. The defects in the surface of the film then lead to further defects in the coated film.
It was found that extrudates (films, sheets and cable sheathings) and molding compounds produced by press molding methods, based on the molding compounds according to the invention, have surfaces with a low concentration of defect sites. A further advantage of the invention is that the thermoplastic molding compounds may readily be processed into extrudates, for example by extrusion, blow molding, cable pultrusion, and into molded articles by press molding methods. The extrudates according to the invention and molded articles produced by press molding methods may then be further processed, printed and/or laser inscripted by conventional techniques, e.g. thermoforming.
The object of the present invention is to provide linear polycarbonate resin having a low fluorescent particle count, making it suitable for extrudates such as films, sheets and cable sheathings having improved surface quality and which may be produced by conventional techniques such as for example extrusion, blow molding or press molding methods from the thermoplastic molding compounds.